The network centralized control scheme is applied in a Long Term Evolution (LTE) system so that both uplink and downlink data of User Equipments (UEs) is transmitted and received under the control of a network, and there is no direct communication link present between one UE and another UE, but communication between them needs to be forwarded and controlled by the network, and no UE is allowed to transmit uplink data by itself, as illustrated in FIG. 1a. 
In the prior art, pure Device to Device (D2D) communication refers to direct communication between one UE and another UE through Bluetooth, Wireless Fidelity (WiFi) and other technologies, and such communication is performed by the UEs per se over unlicensed band resources without being managed and controlled by a network operator and lends itself to very limited application scenarios.
If the D2D communication technology is introduced to a mobile communication operator network, then another D2D communication mode can be applied in which some direct communication are allowed between UEs so that one of the UEs can obtain information directly from the other UE in the D2D communication manner, and these direct communication links can be established under the control or assistance by the network, and D2D communication between the UEs is based upon discovery of a proximate UE.
As illustrated in FIG. 1b, an evolved Node B (simply eNB) establishes uplink/downlink communication links respectively with a UE1 and a UE2 under the control by a Core Network (CN) to configure the UE1 and the UE2 with resources so that both the UEs configured with the resources can perform D2D communication based upon resource configuration information.
As can be apparent, no direct communication between the UEs in the D2D communication manner can be supported in the cellular system in the prior art, and there is such a requisite condition, under which the cellular system can be enabled to support the D2D communication mode between the UEs, that both the D2D communication parties must obtain the D2D communication resource configuration information in advance.